Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional oil well wireline logging, a probe or “sonde” that houses formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
However, wireline logging can generally not be performed while the drilling assembly remains in the borehole. Rather, the drilling assembly must be removed before wireline logging can be performed. As a result, wireline logging may be unsatisfactory in situations where it is desirable to determine and control the position and orientation of the drilling assembly so that the assembly can be steered. Additionally, timely information may be required concerning the nature of the strata being drilled, such as the formation's resistivity, porosity, density and its gamma radiation characteristics. It is also frequently desirable to know other downhole parameters, such as the temperature and the pressure at the base of the borehole, for example. Once this data is gathered at the bottom of the borehole, it is necessary to communicate it to the surface for use and analysis by the driller.
In logging-while-drilling (LWD) systems, sensors or transducers are typically located at the lower end of the drill string. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Typically, the downhole sensors employed in LWD applications are built into a cylindrical drill collar that is positioned close to the drill bit. There are a number of existing telemetry systems that seek to transmit information obtained from the downhole sensors to the surface. Of these, the mud pulse telemetry system is one of the most widely used for LWD applications.
In a mud pulse telemetry system, the drilling mud pressure in the drill string is modulated by means of a valve and control mechanism, generally termed a pulser or mud pulses. The data transmission rate, however, is relatively slow due to pulse spreading, distortion, attenuation, modulation rate limitations, and other disruptive forces, such as the ambient noise in the drill string. A typical pulse rate is less than 10 pulses per second (10 Hz). Given the recent developments in sensing and steering technologies available to the driller, the rate data can be conveyed to the surface in a timely manner, a few bits per second, is sorely inadequate.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.